In blow-molding plastic articles, such as bottles and the like, a substantially-cylindrical preform or parison is first extruded through the orifice of an extrusion die. This type of die generally includes a die head provided with a passageway through which the heated plastic material is forcibly expelled. A mandrel is operatively arranged for controlled axial movement relative to this passageway to vary the cross-sectional annular are of the orifice formed between the mandrel and the walls of the die head surrounding the material passageway. Plasticized material is extruded through the orifice to form a parison or preform.
If the parison is to be subsequently subjected to a blow-molding operation, the outer surface of the object-to-be-formed will ultimately be determined by the internal shape and configuration of the mold. Hence, it is desirable to selectively vary the wall thickness of the parison at different points along its extruded longitudinal extent, in order that sufficient material will be available at the desired locations on the parison, when the parison is subsequently inflated. This is particularly true when the object-to-be-produced is to have an articulated outer shape, but substantially-constant wall thickness.
To produce a parison having the desired wall thickness profile, the mandrel is moved axially relative to the die head during the extrusion cycle to vary the size of the orifice area through which material is extruded. This orifice area (A) is annular, and is proportional to the difference between the square of the effective die passageway diameter (D) and the square of the mandrel diameter (d) at the orifice [(i.e., A=.pi./4)D.sup.2 -d.sup.2)]. The size of the orifice is often expressed in terms of the die gap (G), which is simply the difference between the die head and mandrel diameters (i.e., G=D-d). Thus, the size of the orifice may be expressed in terms of its area (A) or in the terms of the die gap (G), and these two expressions are related to one another, albeit non-linearly. Hence, if the mandrel is moved toward the die head, the width of the die gap will be reduced, less material will be extruded therethrough, and the wall thickness of the parison will be reduced at such locations. On the other hand, if the mandrel is moved away from the die head so as to increase the width of the die gap, a greater volume of material may be extruded therethrough, and the wall thickness of the resulting parison may be greater in such regions. Thus, the mandrel must be capable of controlled movement toward and away from the die head to selectively vary the wall thickness profile of the parison as it is formed.
The wall thickness pattern along the length of the preform, also commonly known as the profile, may, for example, be determined by means of a plurality of potentiometers, each representing a discrete point along the axial length of the parison. In certain cases, there may be as many as twenty-five potentiometers. These may be of the slide-type, and therefore capable of infinite adjustment. The output voltages of the potentiometers are successively sensed during an extrusion cycle, and are supplied to a servoactuator which selectively controls the position of the mandrel relative to the die head as a parison is extruded. Thus, the group of potentiometers, whatever their number, may constitute a wall thickness program generator, and, assuming their individual settings remain unchanged, also serves as a type of memory for storing the desired profile from one extrusion cycle to the next.
The particular settings of the individual potentiometers are normally determined and optimized on an individual basis, simply by trial-and-error. To simplify adjustment, some control circuits incorporate a variable adjustment by which each of the individual output voltages may be modified at the same time. This type of controller is shown and described, for example, in Pamphlet 823D, Systemtechnik fur Blasformmaschinen, Moog GmbH, Boblingen, Federal Republic of Germany (October 1983). This adjustment is somewhat analogous to a common gain in the sense that the output signals of the several individual potentiometers will be multiplied thereby. Hence, if the output voltage of each individual potentiometer is multiplied by a common gain, the output voltages of all potentiometers can be proportionally changed by selectively varying the magnitude of the gain. If the gain is 1.0, then the output voltage of each potentiometer will correspond to its individual setting; if the gain is less than 1.0, then the output voltages of the various potentiometers will be proportionally reduced below their individual settings; and if the gain is greater than 1.0, then the output voltages of the several potentiometers will be proportionally increased above their individual settings. Thus, while the settings of the various potentiometers may be selectively changed on an individual basis, a variation in the common gain will simultaneously affect the output voltages of all potentiometers. Moreover, it is possible to add to these signals, a constant signal representing a basic die gap or minimum orifice area.
This solution suffers a disadvantage if the relationship between the variable program (i.e., the individual output voltages multiplied by the common gain) and the basic or minimum die gap, is fixed. For example, if such relationship is, say 2-to-1, a maximum of one-third of the maximum possible wall thickness can be selected by varying the basic die gap, whereas a maximum of two-thirds of such maximum possible wall thickness can be selected by adjusting the variable program. Hence, the annular extrusion die orifice can not be completely opened by varying the basic gap alone; nor can it be completely opened by varying the adjustable program alone. Thus, if it were desired to decrease a particular set point of the wall thickness program to a value less than that initially set by the basic die gap, the basic die gap would first have to be reduced, and then each setting of the variable program would have to be reset accordingly. Conversely, if it were desired to increase a particular set point to a value greater than the limit of the variable program, then the width of the basic die gap would first have to be increased, and thereafter the variable program would have to be appropriately reduced to accommodate the increase in the basic gap. Hence, readjustments exceeding specific limit values often necessitated resetting all the potentiometers, followed by repetitive trial-and-error techniques to optimize the varied profile.